Defluorination of phosphate rock



Patented Nov. 21, 1950 DEFLUORINATION or rnosrna'rn noon Clinton A. Hollingsworth, Lakeland, Fla., assignor to Coronet Phosphate Company, New York, N. Y., a corporation of New York No Drawing. Application April 26, 1946, Serial No. 665,347

Claims. I

This invention relates to the defluorlnation of phosphate rock and has for its object the provision of a novel and improved method of defluorination.

Fluorine is present in practically all native phosphate rock, in amount varying in the different areas in which it occurs. The common Florida phosphate rock (e. g. pebble rock) usually contains from 3.5 to 4% of fluorine, around 75% bone phosphate of lime (B; P. L.)', around 5% silica, around 5% calcium carbonate, around 2% iron and aluminum oxides, and the balance organic matter and other impurities. The fluorine is commonly believed to be present as calcium fluoride and also combined with the tricalcium phosphate as calcium fluorphosphate or fluorapatite (C3.10F2(PO4) s), and this combination is believed to be largely responsible for the 'ldw'fertilizer emciency of the raw rock (or concentrate), as evidenced by the customary ammonium citrate solubility test. Moreover, the high fluorine content of the raw rock makes it unsuitable as an animal feed or mineral supplement. Generally speaking, a fluorine content in excess of about 0.3%, or over 1 part of fluorine per 30. parts of phosphorus, makes a phosphatic material unsuitable as a mineral supplement.

I have discovered that the fluorine in fluorapatite is replaced by chlorine when the fluorapatitels heated to a temperature of from 1700 to 2200 F. in the presence of a suitablechlorinecontaining agent. The fluorine may be thus replaced to a suflicient extent to permit the use of the defluorinated product as an animal feed or mineral supplement, since the substituted chlorine is non-toxic and unobjectionable. However, the substitution of chlorine for fluorine in fluorapatite does not break the bond of the apatite lattice, and the resulting chlorapatite has the same low I fertilizer eificiency as fluorapatite, I have further found that the chlorapatite can be dechlorlnated by calcination in the presence of water vapor, by substantially the same procedures heretofore proposed for the similar defluorination of fluorapatite, and when thus dechlorinated the phosphatic material has high fertilizer efliciency. I

Based on these discoveries, my present invention, in one aspect, involves subjecting porous agglomerates of finely divided phosphatic material containing fluorapatite to the action of a gaseous chlorine-containing agent at a temperature of at least 1700 F., but not so high that substantial fusion takes place, and thereby replacing with chlorine a substantial amount of the fluorine in the fluorapatite, and thus converting the fluorapatite in large part to chlorapatite. The gaseous chlorine-containing agent should penetrate the entire body or charge of phosphatic material undergoing treatment. The chloridizing treatment is expedited by forming the finely divided phosphatic material into porous nodules which are readily penetrated by the gaseous chlorine-containing agent. The chlorine-containing agent may be chlorine gas, hydrochloric acid (hydrogen chloride) gas, or mixtures thereof, or other suitabe chlorine-containing gas. In another aspect, the invention involves further treatment of the resulting chlorapatite by calcination in the presence of water vapor until the phosphatic material is sufliciently dechlorinated to have high fertilizer efilciency. The calcining temperature of dechlorination is dependent, to some extent, upon the amount of silica present and the tendency of the charge to fuse, varying in general from at least 2500 F. to 3000 F. Temperatures so high as to cause substantial fusion or sintering should be avoided, since such fusion or sintering impedes effective penetration of the water vapor.

In carrying out the invention, the phosphate rock should be in a finely divided state, preferably so that at least passes through a 200 mesh standard Tyler screen and all passes through a 65 mesh screen. While the phosphate rock may be subjected to the chloridizing treatment in its finely divided form, superior results are generally attained by forming the finely divided rock into nodules, pellets, briquets or the like. Nodulizing or the like may be carried out as a preliminary or preparatory operation or may be effected in situ immediately preceding the chloridizing treatment. For example, suitable nodules, generally round in shape and varying in diameter from to 1 inch, may be made by moistening the finely divided rock with water or other suitable liquid and tumbling at room temperature in a rotating cylinder, barrel or the like. From 12 to 18% by weight of water or the like, and generally around 15%, will willciently moisten the dry finely divided rock for producing satisfactory nodules by tumbling. Nodules may also be made by drying an aqueous slurry of the rock, and cutting the dried product into suitably sized cubes or other shapes. Nodules may also be made mechanically by briquetting or the like, although nodules 50 Produced are generally too dense or compact for effective subsequent penetration of the gaseous hlo ine-containing agent. when the chloridwith some substance that volatilizes upon the api zing treatment is carried out in a rotary kiln, plication of heat, such as ammonium carbonate, nodulizing may conveniently be effected in situ or sulphur which burns 011 as sulphur dioxide, or by making a slurry of the finely divided phosone of a large variety of high fusion sulphates.

phate rock with from 40 to 50% by weight of carbonates. halides etc. which lose water of hywater, and heating the slurry and evap ra in dration upon heating. Porosity may also be obthe water in the low perature end of the tained by mechanical means suchas the addirotating kiln. tion of a frothing agent (e. g. soya bean extract,

The chloridizing treatment can be carried out egg albumen etc.) an aqueous slurry of the in any suitable apparatus, such as a rotary kiln, l0 finely divided phosphatic material, and then electric furnace, shaft furnace, sintering machine vigorously agitatin the slurry, whereupon the tc, The multiple hearth shaft furnace disclosed slurry dries with a fluffy or porous structure. in the copending patent applica i n of one of us I prefer, however, to impart porosity to the (Maust) Ser. No. 679,178 filed June 25, 1946, is a nodules by the invention of the copending patent particularly suitable time of app r f r e application of Ernest J. Maust and myself, Ser.

practice of the invention. In whatever apparatus No, 665,348 filed April 26, 1946. In accordance employed, intimate and continuous association of with that invention, a highly effective porosity'is the chloridizin agent with the entire body or imparted to the nodules by including from 5 to charge of phosphatic material undergoing treat- 50% by weight of carbonaceous material in the ment is necessary. As rapidly as chloridizing nodules and eliminating substantially all of the agent is consumed in the operation, fresh chlocarbon of the carbonaceous material by reaction rldizing agent must be available to instantly rewith water vapor at a temperature in excess of place that consumed, and free evolution of any 1800 F. with evolution of the resulting gaseous gaseou products is necessary. Heating in the products. At that temperature, the water vapor presence of the chloridizing agent is thus conreacts with the hot carbon of the carbonaceous tinued until the desired substitution of chlorine material with the evolution of hydrogen and carfor fluorine is attained. In general, the higher bon monoxide, in much the same way that water the temperature the shorter is the required degas is formed. The heat treatment in the prestention period at that temperature to efiect the ence of water vapor is continued until the nodules contemplated defluorination and chlorination. so are decarbonized for'all practical purposes. that With most phosphate rocks a treatment temis until substantially all of the carbon has been perature of from 2000 F. to 2400 F. and a deteneliminated, leaving the nodules with the desired tion period of from 15 to minutes gives exceldegree of porosity. The decarbonizing temperalent results. Too high a temperature may cause ture may advantageously be from 2000 to 2400 such substantial fusion or sintering of the charge 85 F. Too high a temperature should be avoided, as to seriously impede or even prevent eflective since it may cause a loss of phosphorus through penetration of the gaseous chloridizing agent reduction of the phosphatic material by carbon, throughout the entire body thereof, and hence or may cause such substantial fusion or sintering should be avoided. of the nodules as to impede eflective penetration The invention, in its broad aspect, is illustrated 40 of the water vapor throughout the mass or each in the following examples in which the phosphate individual nodule, with the result that all of the rock was a Florida pebble phosphate of which carbon is not eliminated. 0n the other hand. a

the following is a typical analysis: slight amount of sinterlng i advantageous since Per cent it imparts a desired amount of strength to the T ta} P205 35 1o decarbonized porous nodules. Thus. in the case 0 10 of most finely divided phosphate rocks, incipient Insoluble (S 1) slntering takes place at about the same temperaf g A1103 tures at which the water vapor reacts with the ag 70% minus 200 m esh carbonaceous mater and this slight incipient sintering is suflicient to convert the nodules into 7 The finely ground phosphate rock was ma e relatively hard clinkers. Hence, when porosity up to a l rry. dried and cut up into in isattained by the removal of carbon, the clinkered cubes. The cube nodules were introduced in nodules are sufliciently strong to withstand subn el y heated furnace. and hydrochloric sequent handling without disintegration in the acid as and a w re passed through the nodules 85 chloridizing kiln or furnace, as well as in any while maintained at the indicated diflerent subsequent dechlorinating kiln or furnace.

treatment temperatures. Fluorine determinations' were made after the indica d ime in i available for imparting porosity to the nodulized y 1 vols at each treatment temperature. charge. Among these may be mentioned, by way j of example, bituminous or anthracite coal, coke, Percent fluorine with Treatment Tempcracharcoal, lamp black and other forms of carbon,

. Minutes at Treatliquid and solid petroleum products, waste sul- Tempel-ature phite liquor, flour, distillery slops, sawdust,

' ground up grape fruit peelings etc. Solid carbonaceous materials are crushed (when neces- 5 -3g sary) and finely ground, preferably so that at I21 I08 I02 least passes through a 200 mesh standard y g Tyler screen, and substantially all passes through yy t finel divided phosphatic u -1 ,1 a 65 mesh screen. At least 5% by weight of carodulized, the nodules should preferably have a bonaceous m a is q d o impart a y Mme-containing agent may effectively peneenerally fr m 1 up to 40% is preferred. The

, throughout the entire mas of ach inhigher the percentage of carbonaceous material nodule. A desired degree of porosity may initially included in the nodules, the higher will obtained by mixing the phosphatic material be the porosity of the decarbonized nodules.

A wide variety of carbonaceous materials are M degr e of porosity in order that the gaseous efl'ective degree of porosity to the nodules-and The following examples illustrate the ease of chloridizing when porosity is imparted to the nodules by decarbonizing initially included carbonaceous material. The Florida pebble rock, of the approximate analysis hereinbefore mentioned, was mixed with 40% by weight of a high grade (volatile matter about 29%, ash content about 7%) bituminous coal ground to about 52% minus 200 mesh, and the mixture made up into generally round nodules about inch in diameter. The carbon of the coal was then removed by heating the nodules in the presence of water vapor at a temperature of about 2200 F., resulting in very porous and slightly clinkered nodules. The nodules were cooled, and then heated in an electric furnace in contact with a stream of hydrochloric acid gas and air continuously passed through thenodulized charge. Fluorine determinations were made at the different treatment temperatures after various time intervals of treatment.

It will be noted from the foregoing examples that increasing the treatment temperature to 2200 F. very greatly reduces the detention period for substantially complete defluorination. Comparison with the previously recited examples shows that with the charge of highly porous nodules, substantially complete defluorination can be effected at a lower treatment temperature (for the same detention period), or in a shorter time interval (at the same treatment temperature).

In the following examples, porosity was imparted to the nodules by a soya bean foaming agent. The finely ground Florida pebble rock was made into a slurry with water to which the foaming agent was added and the mixture fluffed up, dried and cut up into inch cube nodules.

The porous cube nodules were heated in an electric furnace in an atmosphere of hydrochloric acid gas and air, as in the preceding examples.

Percent fluorine with treatment Minutes at treatment temtemperature 01; f

pernturc 2.l00 F. 2,200 F. 2,300 F.

Percent Total P205 36.26 Insoluble matter (SiO2) 6.00 Fluorine 1.86 F8203 and A1203 4.70

A charge of these porous nodules was then re-- heated in an atmosphere of hydrochloric acid gas and air for 20 minutes at a temperature of 2200 F. The defluorinated (and now chlorinated) nodules, after cooling, analyzed as follows:

Insoluble matter (SiOz) 5.00 FezOa and A1203 4.10 Fluorine .06 Chlorine 5.40

It will be seen from the foregoing analysis that the chloridizing treatment reduces the fluorine content of the phosphatic material to well below the tolerable amount permissible for its use as an animal feed or mineral supplement, and the full availability of the phosphorus content (P205) for this use is shown by the solubility in 0.4% hydrochloric acid (HCl) solution. Additionally, the chloridizing treatment removes some silica as well as some iron and'aluminum. However, the fertilizer efficiency of the chlorinated phosphatic material remains about as low as in the original fluorapatite, as shown by the ammonium citrate solubility. The chlorinated phosphatic material can be dechlorinated and its phosphorus content (P205) made highly available as a fertilizer by the procedures heretofore perfected for defluorinatin phosphate rock by calcination in the presence of water vapor, such, for example, as described in the copending patent application of Ernest J. Maust, Ser. No. 530,156, filed April 8, 1944, issued as Patent No. 2,446,978 on August 10, 1948 and in the applications of Maust and myself Ser. Nos. 665,344, filed April 26, 1946 issued as Patent No. 2,479,389 on August 16, 1949, 665,345 filed April 26, 1946, now abandoned, and 665,346 filed April 26, 1946 issued as Patent No. 2,478,200 on August 9, 1949. Thus. the chlorinated nodules, produced in the foregoir example, were calcined in the presence of wat vapor at a temperature in excess of 2550 F. but not so high that substantial fusion or sintering took place. The analysis of the thus dechlorinated nodules was as follows:

Per cent Total P205 37.46 Availability in 0.4% HCl solution 37.44 Availability in neutral ammonium citrate, A. O. A. C. ofiicial method 35.94 Availability in 2% citric acid 36.50 Fluorine .04 Chlorine .01

While hydrochloric acid gas, preferably in conjunction with air, was the chloridizing agent used in all of the foregoing examples, chlorine gas and water vapor give equally good results. The chemical reaction appears to be merely an interchange of the fluorine and chlorine between the apatite and the chloridizing agent. The fluorine is evolved principally, if not solely, as hydrofluoric acid (HF) gas, and hence the need for water vapor when chlorine gas is the chloridizing agent. The fluorine may be recovered as in the aforementioned processes for defluorinating phosphate rock. However, since for economical operation, any excess chloridizing agent should be recovered and recycled, the gaseous product of the chloridizing treatment is preferably first treated to condense or otherwise appropriately recover the hydrofluoric acid gas, and the residual gaseous product is then recycled through the chioridizing apparatus, or appropriately treated for recovery of its chlorine content in whatever form present. The recovered chlorine may be reused in the chloridizing treatment.

Hydrochloric acid gas and chlorine, for the practice of the invention, may be produced in various ways. For example, hydrochloric acid gas may be generated by the action of sulphuric acid on sodium chloride, or by the decomposition t ammonium chloride by heating. Hydrated magnesium chloride (MgCh-fiHzO) may be decomposed by heat and the resulting chlorinecontaining gas used as the chloridizing agent. Liquid chlorine is a suitabiesource of chlorine gas.

When nodulizing the finely divided phosphatic material, it is advantageous to mix from 0.5 to 2.0% by weight of bentonite with the material in order to impart a desirable degree of hardness and strength to the dried nodules to withstand subsequent handling, and to prevent objectionable dusting of the nodulized charge during chloridizing, especially in a rotary kiln. Some phosphate rocks, such for example as Florida pebble rock and especially the phosphate rocks from North Africa, have when finely ground suflicient natural colloidal constituents to form strong nodules upon drying, and hence require but a. small addition (e. g. 0.5%) of bentonite. On the other hand, phosphate rocks of the apatite type have little or no natural colloidal constituents, and as much as 2% by weight of bentonite may advantageously be mixed with the phosphatic material to impart the desired degree of strength to the dried nodules.

I claim: A

l. The method of defluorinating phosphate rock containing fluorapatite which comprises finely grinding the rock and forming it into porous agglomerates, heating the porous agglomerates o'f finely-ground rock to a temperature of at least 1800 F. but not so high that substantial fusion takes place, heating the'agglomerates to maintain them at such temperature and simultaneously subjecting them to intimate contact with a chloridizing agent selected from the class consisting of hydrochloric acid gas and chlorine gas, replacing the chloridizing agent consumed in the operation as rapidly as it is consumed, continuing the treatment of the agsumed, continuing the treatment of the agglomerates at such temperature in intimate and continuous' contact with chlorine gas and water vapor for a period of at least ten minutes and until the fluorine in the fluorapatite is replaced by chlorine and evolved as hydrofluoric acid gas to an extent such that the fluorine content of the agglomerates is reduced to less than 0.3%, and then withdrawing the resultant chlorapatite from the chloridizing and heating zone, the agglomcrates undergoing treatment being sufllciently porous to permit efiective penetration of the chlorine gas and free escape of the evolved hydrofl-uoric acid.

5. The method of defluorinating phosphate rock containing fluorapatite which comprises finely grinding the rock and formin it into porous agglomerates, heating the porous agglomerates of finely-ground rock at a temperature of from about 2000 F. to 2400" F. in the absence of substantial fusion, heating the agglomerates to maintain them at such temperature and simultaneously subjecting them to intimate contact with hydrochloric acid gas, replacing the hydrochloric acid gas consumed in the operation as rapidly as it is consumed, continuing the treatment of the agglomerates at such temperature in glomerates at such temperature in intimate and continuous contact with the chloridizing agent for a period of at least ten minutes and until the fluorine in the fluorapatite is replaced by chlorine and evolved as hydrofluoric acid gas to an extent such that the fluorine content of the agglomerates is reduced to less than 0.3%, and

then withdrawing the resultant chlorapatite from the chloridizin and heating zone, the agglomerates undergoing treatment being sufllciently porous to permit efl'ective penetration of the chloridizing agent and free escape of the evolved hydro-' intimate and continuous contact with hydrochloric acid gas for a period of at least ten REFERENCES CITED The, following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,111,490 Perino Sept. 22, 1914 1,174,176 Newberry et a1 Mar. 7, 1916 1,396,149 Soper Nov. 8, 1921 1,902,832 Caldwell Mar. 28, 1933 2,061,639 Seyfried Nov. 24, 1936 2,093,176 Tromel Sept. 14, 1937 OTHER REFERENCES Melior, Comprehensive Treatise on Inorganic and Theoretical Chemistry," vol. II, Longmans, New York, 1922, p. 5. 

